The present invention relates to an electron gun for a system wherein electrons are emitted from a cathode or emitter by an electric field in an electron microscope or the like.
The field emission electron gun is based on a method in which a high electric field (of, for example, 10.sup.7 V/cm) is applied to a pointed cathode (having a radius of curvature of, for example, 1,000 A or 100 nm), thereby to emit electrons owing to the tunnel effect. It makes it possible to fabricate an electron microscope which has a much higher brightness and smaller electron source than with the conventional thermal electron emission and which exhibits a high resolving power.
In case of utilizing the field emission electron gun for the electron microscope, it is desired to properly use two levels of energies of low energy (for example, approximately 5 kV) and high energy (for example, approximately 10 kV or above) as the energies of an electron beam to impinge on a sample. This is because the low energy brings forth the advantages of preventing the charge-up of the sample, permitting the observation of a very shallow surface portion of the sample, etc., while the high energy can reduce astigmation and makes it possible to realize an electron microscope of high resolving power.
With the field emission electron gun, however, the emission electron current is proportional to the applied electric field, so that the high energy (for example, approximately 10 kV or above) results in increasing the emission electron current and in shortening the lifetime of the cathode.
In order to avoid this drawback, an electrode of the Butler type has heretofore been employed. FIG. 1 is a view for explaining a field emission electron gun which employs the Butler type electrode. A drawing-out voltage V.sub.A (for example, approximately 5 kV) is applied between a cathode 1 and a first anode 2 by a D.C. power supply 3, to cause the electron emission from the cathode 1. Emitted electrons 9 are accelerated into an electron beam of high energy (for example, 15 kV) by a second anode 5 to which an accelerating voltage V.sub.B is applied by a D.C. power supply 4. The electron beam is focused and projected on a sample 7 by an electromagnetic lens 6. In the figure, numeral 8 designates a vacuum vessel.
This method has the disadvantage that since the first anode 2 and the second anode 5 have the lens action, they form a cause for astigmatism, and such disadvantage in handling that the centering, in the case of the change-over between the high energy and low energy is difficult. Besides, with this method, the structure becomes complicated, and two high-voltage sources are required.